The present invention relates to a solid detergent composition for use in automatic or mechanical dishwashing machines.
Detergent compositions suitable for use in mechanically washing dishes, glasses, eating and cooking utensils are extremely difficult to formulate because of the many influencing factors encountered which are not encountered in providing detergent compositions for other uses.
For example, in a dishwashing machine food soil is removed in part by mechanical action and partly by physico-chemical influences. The mechanical action normally takes place when the soiled surfaces of the load are deluged with wash liquor. Among the physico-chemical factors that affect the cleaning of dishes and cutlery are wetting, emulsification, adhesion of soiled substrate, alkalinity, oxidation potential, soil suspension, anti-flocculation and foam control.
More specifically, food soil may be present in a variety of states. Some foodstuffs are oxidized, polymerized or partially decomposed by cooking, and this will affect the ease with which they may be removed. In addition, generally the main foodstuffs are proteins, phospholipids, triglycerides and carbohydrates and each of these may respond in a different way to physical and chemical conditions. Also the foodstuff may coat widely different substrates. For example, glass, glazes (plain and pigmented) present on pottery and chinaware, metals and plastics present another problem with regard to deleterious effects due to the washing ingredients.
Considering further some of the problems encountered in formulating dishwashing compositions, it is known that in all mechanical dishwashing equipment a serious problem results from "aeration" and "foaming" of the washing solution or rinsing water. The foam is produced primarily by the accumulation of protein food soils (such as egg solids and milk solids) during the various cycles as the dishes are washed. These materials have a natural tendency to foam. Also, since most dishwashing detergents are composed of inorganic alkaline salts, the fatty food soils become saponified in the hot solution and produce copious foam in the machine, even though the inorganic dishwashing detergents do not foam in themselves. The foam not only causes the machine to overflow, but cushions and impedes the mechanical operation of the machine to the extent that performance is measurably decreased.
Consequently, it is generally recognized that the high-foaming conventional anionic surfactants and those having anionic properties are not useful in detergent formulations proposed for use in automatic dishwashers and in place thereof the art has suggested the use of low-foaming nonionics.
In addition to a nonionic surfactant, detergent compositions for use in mechanical dishwashing equipment have invariably contained an alkaline sequestrant builder salt. Utilization of a strong sequestrant or chelate builder is necessary to remove hardness ions, buffer the solution at a pH conducive to good washing, and act as an anti-redeposition agent for soil. Complete, or even partial removal of this material normally results in badly spotted and filmed dishes, glasses, etc. More particularly, the detergent compositions for use in mechanical dishwashing according to the prior art contain from about 20% to 80% condensed phosphates. When used as the sole chelating agent it is necessary to use at least 40% phosphate in order to obtain efficient results, particularly with respect to preventing spotting and filming of glasses.
Of late, attempts by us and others have been made to formulate low-phosphate or phosphate-free products because of potential problems of eutrophication. It is noteworthy that all these attempts have apparently been based upon the replacement of the usual tripolyphosphate sequestrant builder with nonphosphate substitutes. Sodium nitrilotriacetate (NTA) has been an obvious choice, since it sequesters hardness ions equal to or better than the phosphates. Examples of such formulations are given in U.S. Pat. No. 3,544,473. It may be noted that the formulations of the prior art contain high levels of NTA or combinations of NTA with other sequestrant builders.
The present invention affords efficient dishwashing compositions which can be formulated without the need for a builder and particularly provides nonphosphorus formulations which are highly effective and do not present environmental problems.